Radiation image pickup tube



Ap 1964 G. w. GOETZE ETAL RADIATION IMAGE PICKUP TUBE Filed April 28, 1961 Fig. I.

Fig. 3.

SATURATION TIME INVENTORS Gerhard W. Goetze 8 Helmut Komer. @214. Kg ATTORNEY Fig.4.

. n. XWZ? United States Patent 3,128,406 RADIATION IMAGE PICKUP TUBE Gerhard W. Goetze and Helrnut Kanter, Monroeville, la., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Apr. 28, 1961, Ser. No. 106,222 9 Claims. (Cl. 313-65) This invention relates to an image device and more particularly to those devices in which a radiation image is received on an input screen and an output signal is derived therefrom. This invention is particularly useful in direct view imaging tubes in which a radiation image is directed onto an input screen and an output image is displayed on an output screen which is increased in brightness and/or contrast enhancement over the input image.

Another application of the invention is in a camera type tube in which the input radiation image is converted into an electron image and then directed onto a storage member with contrast enhancement. This resulting stored image on the storage member is read out to derive a video signal representative of the radiation image directed onto an input screen with enhanced contrast. The video signal may then be displayed on a conventional display device.

In the direct view type tube, there are several ways of obtaining intensification of the input image. One method of obtaining intensification is the utilization of a storage electrode positioned between the input screen and the output screen. In this type of device, an apertured storage grid is utilized and a charge image is developed on the storage grid corresponding to the input image. The apertured storage grid is then flooded with a uniform high intensity electron beam and this beam is modulated by the charge on the apertured storage grid to provide an intensified image. Another method of obtaining intensification of an input image is by utilizing amplifying structures positioned between the input screen and the output screen. Such a device is described in US. Patent 2,905,844 by E. J. Sternglass entitled Electron Discharge Device issued September 22, 1959 and assigned to the same assignee as the present invention. In the abovementioned patent, transmissive type dynodes comprising a layer of a secondary electron emitting material are provided in which the primary electrons impinge on one surface of the dynode and secondary electrons are emitted from the opposite surface of the dynode. In those cases where extremely low light level scenes are viewed, it would be desirable to amplify the input signal, increase the contrast and also utilize the integrating properties of a storage tube in order to obtain an output sufiiciently bright to be viewed by the human eye.

Accordingly, it is an object of the present invention to provide an improved image device.

It is another object to provide an improved image device capable of contrast enhancement of an input signal.

It is another object to provide an improved image capable of intensifying an input signal.

It is another object to provide an image intensifying device capable of storing image information for a long period of time.

These and other objects are effected by our invention as will be apparent from the following description taken in accordance with the acompanying drawings, through out which like reference characters indicate like parts, and in which:

FIGURE 1 is a diagrammatic view of an image device embodying our invention;

FIG. 2 is an enlarged partial sectional view of the storage electrode utilized in FIG. 1;

FIG. 3 is a graphical representation of imput current to a storage electrode; and

3,128,406 Patented Apr. 7, 1964 ice FIG. 4 is a graphical representation of the yield of a storage electrode.

Referring in detail to FIG. 1, a vacuum tight enclosure or envelope 10 is provided of a suitable material such as glass. The evacuated enclosure 10 includes an elongated tubular portion 12 with an input window 14 closing off one end of the tubular member 12 and an output window 16 closing off the other end of the tubular portion 12. The windows 14 and 16 are also of suitable material such as glass capable of transmission of the input and output radiations. On the inner surface of the input window 14, there is provided an electrically conductive coating 18. The coating 18 is transmissive to the input radiation and may be of a material such as tin oxide. A layer 20 of a suitable photoemissive material such as cesium antimony is provided on the coating 13. The inner surface of the gas.

output window 16 is provided with a suitable light transmissive electrically conductive coating 22 of a suitable material such as tin oxide with a coating 24 of a suitable fluorescent material such as zinc cadmium sulfide provided thereon. The photoemissive layer 20 is responsive to a radiation image directed thereon and emits an electron image corresponding to the radiation image. The fluorescent material in layer 24 emits light in the visible region in response to electron bombardment.

Positioned between the input window 14 and the output window 16 are a plurality of storage electrodes 30. The number of electrodes 30 desired depends on the intensification and/or enhanced contrast desired and only one may be required in some applications or for simple storage applications. The structure of one suitable electrode 30 is illustrated in FIG. 2. The storage electrode 30 consists of a suitable electrically conductive continuous support layer 32 such as aluminum with a highly insulating continuous layer 34 deposited thereon of a suitable material such as aluminum oxide. A continuous layer 36 of a suitable insulating secondary emissive material such as barium fluoride is provided on the layer 34. Other simple alkaline or alkaline earth metal compounds may be used for the layer 36 such as potassium chloride and magnesium oxide. The layer 36 is deposited as a smoke or porous type layer. It may also be desirable in some applications to provide a grid 40 adjacent the layer 36.

A suitable accelerating voltage is provided between the photoemitter 20 and the first storage electrode 30 by means of a battery 31 connected between the conductive coatings 18 and 32. An accelerating voltage is also provided between adjacent storage electrodes 30 by a suitable voltage source illustrated as a battery 33 connected between the coatings 32. A battery 35 connected between the conductive coating 32 of the last storage electrode 30 and the coating 22 provides the necessary acceleration voltage between the last storage electrode 30 and the output phosphor 24.

A specific example of a suitable storage electrode 30 in accordance with the present invention and a method of forming such a structure will now be described. The support layer 32 is of aluminum foil of a suitable thickness secured to a support ring of a suitable material such as Inconel. The thickness of the aluminum support layer 32 should be about 1000 angstroms for an electrode diameter of about one inch. The aluminum foil layer 32 is then exposed to an oxidizing atmosphere to provide the coating 34 of aluminum oxide of about 200 angstroms in thickness on the surface of the aluminum support 32. The structure is then placed in a bell jar having an atmosphere of approximately 1 millimeter mercury of argon A tantalum boat is also disposed in the bell jar. The boat is provided with a resistive heating element and the boat contains about 16 milligrams of barium fluoride in solid or chunk form. The boat is then placed at a distance of approximately 3 inches below the dynode support. Current is applied to the resistive heating element and heating continues until the barium fluoride has just melted, at which temperature the material is maintained. This temperature is considerably less than the melting point of the material which at atmospheric pressure is about 1280 C. The vapor pressure of barium fluoride at its melting point under such conditions is found suflicient to cause vaporization of the material at a sufficient rate. The barium fluoride is evaporated to completion and it is found that the area density of the evaporated barium fluoride on the aluminum oxide layer 34 is approximately 87 micrograms per square centimeter. Such a layer has a thickness of approximately 20 microns. Therefore, it is seen that while barium fluoride has a bulk density of about 4.838 grams per cubic centimeter, a spongy or porous layer formed in the above manner has a density of the order of about 0.04 gram per cubic centimeter. The porous layer 36 provides a layer which has a low bombardment induced conductivity. If the transverse resistivity of the layer 36 is suflieient to prevent charge leakage to the base 32, then the layer 34 may not be necessary.

In the operation of the device, a radiation image is projected onto the photoemissive layer 20 which in turn generates an electron image corresponding to the radiation image and the electrons are accelerated by a positive potential of about 4 kilovolts provided by the source 31 and bombard the first storage electrode 30. In the embodiment shown an image is projected onto the large area cathode. If desired a flying spot scanner modulated with image information could be utilized to excite the cathode. Also a scanning electron beam of small area could be substituted for the large area cathode. The electron beam could then be modulated with the image information.

A suitable focusing system may be provided around the envelope for focusing the electrons between the electrodes within the envelope. In a specific embodiment, the focusing device is shown as a permanent magnet 37 to provide a longitudinal field within the envelope. The electrons emitted from the photoemissive surface 20 are accelerated and pass through the thin aluminum support layer 32 of the first storage electrode 30 into the aluminum oxide and barium fluoride layers 34 and 36. The acceleration voltage should be adjusted such that the primary electrons from the photoemissive layer 20 substantially penetrate the entire storage electrode 30 but do not pass on through the storage electrode 30. The primary electrons bombaring the storage electrode 30 will cause a secondary electron yield from the surface of the layer 36 greater than the number of primaries striking the layer 32. It is also observed that the secondary electron yield of the storage electrode 3% depends on the potential between the emissive surface of layer 36 and the support layer 32. The surface areas with a given positive potential have a larger secondary electron yield and the rate of rise of yield with time is increased over areas with less or no positive potential thereon. This surface potential is produced by the secondary emission process itself in the case where more secondaries leave the emitter than primaries bombard the electrode. The surface potential depends furthermore on the current density of the incoming primaries. The larger the current density the greater will be the rate of rise of yield with time. It has been observed in this type of device that the image current impinging on the storage electrode 30 will create a charge image on the surface of layer 36 corresponding to the electron image but with enhanced contrast. The secondary yield of the emitter and the rate of rise with time changes according to the charge pattern. Consequently, the ratio of secondary electrons emitted from bright to dark image portions will be increased. It is obvious that while this increase might be small for only a single secondary electron multiplier stage 36 a sequence of dynodes 30 can lead to considerable contrast enhancement.

The emission from the electrode 30 ceases on removal of the electron excitation. The operating condition of the electrode 3% must be such that there exists a stable and unique relationship between the yield and the surface charge accumulated. Another way to specify this condition is to characterize it by a reproducible proportional relation between yield and charge. The operating condition must be such that instability does not take place. This can be accomplished by selecting a film that has a saturation value of yield as indicated in FIG. 4. It is also possible to utilize the grid 40 to limit the potential to which the surface of layer 36 can rise. The grid 4%) should be held at a voltage of about 10 to 200 volts positive with respect to the conductive coating 32.

Furthermore, the storage electrode 30 also has storage capabilities. Once a charge pattern is imposed on the surface of layer 36, the image source may be removed. The input photo cathode 20 may now be illuminated uniformly by an auxiliary light source 50. The constant or uniform photo current generated by the cathode 20 and in turn directed onto the storage electrode 36 will be modulated by the charge pattern on the storage electrode 36 due to the variation in yield similar to the grid action in a common electron tube. Again the greater the surface potential the greater will be the secondary emission. It is important that the layers 34 and 36 have a low value of electron bombardment induced conductivity to result in a net positive charge being produced during bombardment. If only the layer 36 is utilized on the conductive layer 32, then it must also have a low value of electron bombardment induced conductivity. The use of the storage effect provides a device in which the light emission of the :output phosphor 24 can be adjusted to a value at which a detecting film placed adjacent thereto may be blackened to its largest contrast. This method can be used in low light level imaging, where the output intensity is too weak to expose the film in a reasonable time. In this case, the image is used just to build up a charge on the electrodes 30. The output energy necessary for suifioient blackening, is supplied by an auxiliary light source uniformly illuminating the intensifier input. Again a flying spot scanner or flooding gun may be used in the reading operations.

The electrode providing the contrast increase in the image intensifier is the electrode 30 consisting in general of the conducting support 32 with the highly insulating layer 34 on one surface which is covered with the insulating deposit 36 of sufiicient low density to provide secondary emission. The density of the secondary emitter layer 36 must be chosen such that the avalanche process, leading to the desired high yield, is dependent on the surface charge. In case the low density deposit 36 has suificient high resistivity, the insulating layer 34 may be omitted. The electrons emitted from the last storage structure 30 are accelerated by a suitable potential of about 15 kilov'olts to the output screen and the electrons bombard the surface catusing the phosphor layer 24 to emit light corresponding to the radiation image directed onto the input screen.

A simple example of the operation of the device can be best illustrated by reference to FIGS. 3 and 4. The radiation input to the tube will consist of a signal represented by 1 in FIG. 3. For example, the image of an illuminated slit is projected onto the photoemitter 20 which results in a current I from the photoemitter 20. A certain background resulting in a current I is also present. During the storage time T shown in FIG. 4 the yield 6 increases from 6 to 6 for those areas bombarded by the current 1 In the same time, the yield 6 increases from 6 to 6 for the rest of the electrode bombarded by the current 1 The rate of rise of yield is greater for I than for I The input image is removed at the end of the storage time. This storage time might be of the order of 20 hours with an input of -12 amperes/cm It is obvious that contrast enhancement is obtained at the end of the storage cycle as the output contrast will be enhanced by the factor 6 /6 This is important in itself, but one may also read out the stored image. After having removed the input, the readout cycle is performed. The photosurface is illuminated by source 50 so that a high output current is o tained as represented by I The current I is large compared to 1 During the readout cycle the yield 5 increases to 6 while the yield 5 increases to 5 The yield is greater for the areas originally excited by 1 than I due to the greater field thereon and the output contrast depends primarily on the storage time. The readout time AT is very short compared to the storage time and may be less than a minute.

There is an upper limit for the output contrast. It is obvious that the maximum output contrast can never be greater than 6 max/6 The ratio may be increased by priming the storage target with a negative charge. It should be again noted that the secondary emissive material should give a maximum change in yield with charge under electronbombardrnent. It is also important that the layer 36 have i-o'w bombardment induced conductivity. Thus a charge once established on the surface of the emitter affects the yield to the full amount and is not diminished by time or further information storage.

The pattern stored on the storage electrode may be removed or erased by directing electrons onto the surface of the secondary emissive layer 3-6. By proper adjustment of the voltages, a portion of the primary electrons may he made to penetrate through the dynode. By applying a negative voltage on the following electrode, the electrons may be directed back onto the surface of the secondary emissive layer 36 and erase the charge image thereon. It is also possible to erase the stored pattern by heating the storage electrodes and thereby reduce the resistance. Another method is to reduce the velocity of the impinging electrons such that the yield is less than .unity.

While we have shown our invention in only a few forms, it Will be obvious to those skilled in the art that it is not so limited but is susceptible to various other changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

-1. An electrical device comprising a target for electrons including a continuous layer of electrically conductive material, and a layer of secondary emissive material deposited on one surface of said conductive layer, said secondary emissive material exhibiting the property of increasing rate of yield of secondary electrons with time at a constant primary current, means for directing an electron image onto the exposed surface of said conductive layer of suffieient energy to penetrate through said conductive layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of said secondary emissive layer such that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having the greater electron excitation such that a greater contrast differential is obtained, and a display screen for receiving the secondary electrons from said target to produce a visible image of enhanced contrast with respect to the contrast of said electron image directed onto said target.

2. A direct view storage tube comprising an input screen of photoemissive material which emits an electron beam in response to a radiation image directed thereon, an output screen spaced from said input screen and having a layer of phosphor material thereon capable of emission of light in response to electron bombardment, a storage electrode positioned between said input screen and said output screen, said storage electrode comprising a continuous layer of electrically conductive material, and a continuous layer of secondary emissive material positioned on the surface of said conductive layer facing said output screen, meansfor directing a radiation image to be stored onto said input screen to generate an electron image corresponding to said radiation image, means for accelerating said electron image to bombard said storage electrode and penetrate said conductive electrode and enter said secondary emissive layer, said secondary emissive layer exhibiting the property of emitting electrons from the exposed surface at a ratio greater than one and the rate of yield of said electrons increasing with time at a constant primary bombardment, means for maintaining the stored radiation pattern from said input screen for a substantial length of time to produce a charge image on the exposed surface of said secondary electron emissive layer of enhanced contrast over said input radiation image, and means for directing an electron beam of uniform intensity over the exposed surface of said conductive layer to generate secondary electrons from the exposed surface of said secondary electron emissive layer corresponding to said charge image and means for accelerating said secondary electrons to bombard said output screen to provide an intensified image of enhanced contrast.

3. A device for the storage of electrical signals comprising a target for electrons including a continuous layer of electrically conductive material, a layer of insulating material deposited on one surface of said conductive layer and a layer of secondary emissive material deposited on the surface of said insulating layer, said secondary emissive material exhibiting the property of increasing rate of yield of secondary electrons with time at a constant primary current, means for directing, an electron image to be stored onto the exposed surface of said conductive layer of sufficient energy to penetrate through said conductive layer, said insulating layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of said secondary electron emissive layer so that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having the greater excitation such that a greater contrast differential is obtained in accordance with the time of excitation, said storage electrode storing said information in the form of a charge pattern on the surface of said secondary emissive material and means for generating a uniform electron beam of greater density than said storage information so as to obtain an intensified electron image of increased contrast from the secondary emissive surface of said storage electrode.

4. A direct view storage tube comprising an input screen of photoemissive material which emits an electron beam in response to a radiation image directed thereon, an output screen spaced from said input screen and having a layer of phosphor material thereon capable of emission of light in response to electron bombardment, a storage electrode positioned between said input screen and said output screen, said storage electrode comprising a continuous layer of electrical conductive material, a layer of insulating material provided on the surface of said conductive layer facing said output screen and a layer of secondary emissive material positioned on the surface of said insulating layer facing said output screen, means for directing a radiation image for a predetermined time onto said input screen to be stored to generate an electron image corresponding to said radiation image, means for accelerating said electron image to a predetermined energy to bombard said storage electrode and store an image thereon due to secondary emission from the exposed surface of said secondary emissive layer, said secondary emissive layer exhibiting the property of emitting electrons at a ratio greater than one in response to bombardment at said predetermined energy and the rate of yield of said electrons increasing with time at a constant primary bombardment, means for flooding said input screen with uniform illumination of greater intensity than said radiation for a length of time less than said predetermined time to provide an intensified light image from said output screen of greater contrast.

5. An electrical device comprising a target for electrons including a continuous layer of electrically conductive material, and a layer of secondary emissive material deposited on one surface of said conductive layer, said secondary emissive material exhibiting the property of increasing rate of yield of secondary electrons with time at a constant primary current, means for directing an electron beam onto the exposed surface of said conductive layer of sufiicient energy to penetrate through said conductive layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of said secondary emissive layer such that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having the greater electron excitation such that a greater contrast differential is obtained and thereby establish a charge on the exposed surface of said secondary emissive layer representative of the electron excitation directed thereon and means for generating and directing a reading electron beam onto said target to derive electrical signal therefrom representative of the charge pattern established thereon.

6. An electrical device comprising a target for electrons including a continuous layer of electrically conductive material, a porous layer of secondary emissive material deposited on one surface of said conductive layer, said secondary emissive material exhibiting the property of increasing rate of yield of secondary electrons with time at a constant primary current due to establishment of a field across said layer of secondary emissive material, means for limiting the field across said secondary emissive layer to obtain a reproducible proportional relation between the secondary emission and the field across said layer, means for directing a first electron beam onto an exposed surface of said conductive layer of sufficient energy to penetrate through said conductive layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of the secondary emissive layer such that the secondary yield from said second ary emissive layer increases at a greater rate in those areas subject to greater electron excitation so that a greater contrast differential is obtained and thereby establish a charge pattern on the exposed surface of said secondary emissive layer representative of the electron excitation directed thereon and means for generating and directing a second electron beam onto the target to derive an electrical signal therefrom representative of the charge pattern established thereon.

7. An electrical device comprising a target for electrons including a continuous layer of electrically conductive material, a porous layer of secondary emissive material deposited on said conductive layer which exhibits the property of emission of secondary electrons from one surface of said layer in response to bombardment of the other surface of said layer with primary electrons and an increasing ratio of yield of secondary electrons with a constant primary electron bombardment due to establishment of a field across said layer, an electrically conducting means provided adjacent an exposed surface of said secondary emissive layer for limiting the field across said secondary emissive layer to obtain a reproducible proportional relation between the secondary emission and the field across said layer, means for directing a writing electron beam onto an exposed surface of said conductive layer of sufficient energy to penetrate through said conductive layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of said secondary emissive layer such that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having a greater electron excitation such that a greater contrast differential is obtained and thereby establishes a charge on the exposed surface of said secondary emissive layer representative of the elec- 8 tron excitation and directed thereon and means for generating and directing a reading electron beam onto said target to derive an electrical signal therefrom representative of the charge pattern established thereon.

8. An electrical device comprising a target for electrons including a continuous layer of electrically conductive material, a layer of secondary emissive material deposited on one surface of said conductive layer, said secondary emissive material exhibiting the property of emission of secondary electrons from one surface of said layer in response to bombardment of the other surface of said layer with primary electrons and an increasing ratio of yield of secondary electrons with a constant primary electron bombardment due to an establishment of field across said layer, means for limiting the field across said secondary emissive layer to obtain a reproducible proportional relation between the secondary emission and the field across said layer, said means including an electrically conductive grid member positioned in close proximity to said secondary emissive layer, means for directing a writing electron beam onto an exposed surface of said electrically conductive layer of sufiicient energy to penetrate through said conductive layer and into said secondary emissive layer to generate secondary electrons from the exposed surface of said secondary emissive layer such that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having the greater electron excitation to provide a greater contrast differential and establishment of a charge on the exposed surface of said secondary emissive layer representative of the electron excitation directed thereon and means for generating and directing a reading electron beam onto said target to derive an electrical signal therefrom representative of the charge pattern established thereon.

9. An electrical device comprising a target for electrons including a continuous layer of material of a porous nature which exhibits the property of emission of secondary electrons from one surface of said layer in response to bombardment of the other surface of said layer with primary electrons and an increasing ratio of yield of secondary electrons with a constant primary electron bombardment due to an establishment of a field across said layer, an electrically conductive means provided on the side of said target facing an exposed surface of said secondary emissive layer for limiting the field across said secondary emissive layer to obtain a reproducible proportional relation between the secondary emission and the field across said layer, means for directing a writing electron beam onto an exposed surface of said conductive layer of sufficient energy to penetrate through said conductive layer and to said secondary emissive layer to generate secondary electrons from the exposed surface of the secondary emissive layer such that the secondary yield from said secondary emissive layer increases at a greater rate in those areas having the greater electron excitation such that a greater contrast differential is obtained and thereby establish a charge pattern on exposed surface of said secondary emissive layer representative of the electron excitation by said writing beam and means directing a reading electron beam onto the exposed surface of said conductive layer of sulficient energy to penetrate through said conductive layer and into said secondary emissive layer to generate a secondary emissive electron image representative of the charge pattern established on said target by said Writing electron beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,254,617 McGee Sept. 2, 1941 2,518,434 Lubszynski Aug. 8, 1950 2,587,830 Freeman Mar- 2 2,822,493 Harsh Feb. 4, 1958 

2. A DIRECT VIEW STORAGE TUBE COMPRISING AN INPUT SCREEN OF PHOTOEMISSIVE MATERIAL WHICH EMITS AN ELECTRON BEAM IN RESPONSE TO A RADIATION IMAGE DIRECTED THEREON, AN OUTPUT SCREEN SPACED FROM SAID INPUT SCREEN AND HAVING A LAYER OF PHOSPHOR MATERIAL THEREON CAPABLE OF EMISSION OF LIGHT IN RESPONSE TO ELECTRON BOMBARDMENT, A STORAGE ELECTRODE POSITIONED BETWEEN SAID INPUT SCREEN AND SAID OUTPUT SCREEN, SAID STORAGE ELECTRODE COMPRISING A CONTINUOUS LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL, AND A CONTINUOUS LAYER OF SECONDARY EMISSIVE MATERIAL POSITIONED ON THE SURFACE OF SAID CONDUCTIVE LAYER FACING SAID OUTPUT SCREEN, MEANS FOR DIRECTING A RADIATION IMAGE TO BE STORED ONTO SAID INPUT SCREEN TO GENERATE AN ELECTRON IMAGE CORRESPONDING TO SAID RADIATION IMAGE, MEANS FOR ACCELERATING SAID ELECTRON IMAGE TO BOMBARD SAID STORAGE ELECTRODE AND PENETRATE SAID CONDUCTIVE ELECTRODE AND ENTER SAID SECONDARY EMISSIVE LAYER, SAID SECONDARY EMISSIVE LAYER EXHIBITING THE PROPERTY OF EMITTING ELECTRONS FROM THE EXPOSED SURFACE AT A RATIO GREATER THAN ONE AND THE RATE OF YIELD OF SAID ELECTRONS INCREASING WITH TIME AT A CONSTANT PRIMARY BOMBARDMENT, MEANS FOR MAINTAINING THE STORED RADIATION PATTERN FROM SAID INPUT SCREEN FOR A SUBSTANTIAL LENGTH OF TIME TO PRODUCE A CHARGE IMAGE ON THE EXPOSED SURFACE OF SAID SECONDARY ELECTRON EMISSIVE LAYER OF ENHANCED CONTRAST OVER SAID INPUT RADIATION IMAGE, AND MEANS FOR DIRECTING AN ELECTRON BEAM OF UNIFORM INTENSITY OVER THE EXPOSED SURFACE OF SAID CONDUCTIVE LAYER TO GENERATE SECONDARY ELECTRONS FROM THE EXPOSED SURFACE OF SAID SECONDARY ELECTRON EMISSIVE LAYER CORRESPONDING TO SAID CHARGE IMAGE AND MEANS FOR ACCELERATING SAID SECONDARY ELECTRONS TO BOMBARD SAID OUTPUT SCREEN TO PROVIDE AN INTENSIFIED IMAGE OF ENHANCED CONTRAST. 